1. Field of the Invention
This invention relates to the production of shale oil and more particularly to the construction of an in-situ retort and production of shale oil from such retort.
2. Description of the Prior Art
A method of retorting oil shale that has been proposed as particularly advantageous for recovery of shale oil from oil shale deposits covered by an overburden of substantial thickness is in-situ retorting of the oil shale. In that process, an in-situ retort is formed by blasting oil shale to break it into rubble that fills the in-situ retort. Retorting is accomplished by heating the rubblized oil shale to a temperature, generally above 800.degree. F., at which kerogen in the oil shale is converted to shale oil. Heat for the retorting is provided by heating a portion of the oil shale in the retort to an ignition temperature and passing air through the heated portion of the oil shale to continue combustion of carbonaceous material in the oil shale. Gaseous combustion products travel through the in-situ retort to its outlet and in doing so heat oil shale ahead of the combustion front to a temperature at which conversion of the kerogen to the shale oil occurs. The liberated shale oil is delivered to the surface by any suitable means.
Rubblization of the oil shale to form an in-situ retort is necessary because oil shale has a very low permeability which precludes forcing retorting gases through the oil shale at a rate adequate to maintain a combustion front. Rubblization is accomplished by removing by any desired mining method a portion of the oil shale to provide open space and thereafter blasting shale adjacent the open space space to break it into particles to fill the open space. While the open space required for rubblization has been described as less than forty percent of the retort volume, in fact, the open space actually used is substantially less. For example, in U.S. Pat. No. 4,043,597 of French, it is stated that the total volume of the voids should be less than about 40 percent of the volume of the retort; however, that patent states that the void space should be less than 20 percent and is preferably about 15 percent of the volume of the retort. (Throughout the specification and claims the term "open" space designates space devoid of shale outside the boundaries of the rubblized shale, and the term "void" space designates space between shale particles within the rubblized mass.) In U.S. Pat. No. 3,661,423 of Garrett, it is stated that the void space may be 5 to 25 percent of the volume of the retort but is preferably 5 to 15 percent of the retort volume. French describes the pieces of rubblized shale as being wedged together tightly. Garrett describes the rubblized shale as being packed. It has been recommended heretofore that the void space be as low as possible to minimize the amount of mining of oil shale necessary to provide void space.
In the rubblization procedures heretofore described, the amount of open space provided is such that the retort will be filled at the completion of the rubblization and is adequate only to permit enough movement of shale pieces relative to one another to result in fragmentation rather than merely fracturing of the oil shale. By fracturing is meant merely forming a crack in the formation; fragmentation means forming a plurality of interconnecting fractures that break the shale into a plurality of particles. The movement that occurs on the blasting in the methods heretofore used is limited by adjacent pieces of the oil shale so that the movement is essentially a slight moving apart of the pieces of shale with only enough displacement of the pieces of shale to prevent mating as the pieces of shale come to rest after the blasting. In fact, open space permitting the desired fragmentation is derived in the processes heretofore available in part from compressing, wedging or jamming previously rubblized material into a more tightly packed condition. The permeability of such rubblized structures although lower than desirable is adequate to allow some flow of gases through the retort but uniform permeability throughout the retort is not obtained. Because of the nonuniformity of the permeability, channeling of fluids through the retort during in-situ combustion occurs with resultant bypassing of portions of the oil shale and lower yields of shale oil. Moreover, while the permeability is adequate to allow combustion to proceed, the relatively low permeability results in high pressure drop through the retort during the retorting and high costs for circulation of combustion air and retort gases.
In an effort to improve the uniformity of the permeability in the retort, rather elaborate and expensive steps have been taken during the rubblization. For example, in U.S. Pat. No. 4,043,598 of French, an open space is formed at vertical intervals of about 195 feet and unfragmented shale between the open spaces is blasted into the open space. At each open space, it is necessary to construct withdrawal drifts communicating with apparatus for lifting the mined oil shale to the surface. In a retort 1,000 feet high having five intervals of approximately 200 feet, it would be necessary to provide rock handling facilities at five levels with consequent very high mining costs. In spite of the steps taken in an attempt to improve the uniformity of the permeability of a retort, substantial channeling and bypassing still occur.